Fixed handling of zones (import/read)

- bnc#898659
- Checking for system zones was moved to separate function
- Extended list of zones marked as system (internal) ones with (0\.)+ip6.arpa
- Not allowing to edit system zones (belong to bind package)
- Added check for 'bind' package to be installed before writing the configuration
- Marking all imported zones as 'modified' to be written later
- Marking all imported non-system zones as 'is_new' to create a zone file for them
- Flushing /etc/named.conf cache 'after' writing zones (instead of 'before')
- Using "" as the default NETCONFIG_DNS_POLICY (instead of 0)
Well, I'm fixing and old "fix" :) It used to skip writing all system zones. Now they are written if they are marked as modified (but not in case of LDAP in use). I'll mention that in changes file.